The chemicals used for pulping of wood in a Kraft pulp mill are recovered in the Kraft pulp mill chemical recovery cycle. Various losses of chemicals exist throughout the cycle, so makeup chemicals are required. One common source of chemical makeup for mills that generate chlorine dioxide onsite for use in the pulp bleaching process is the effluent from the chlorine dioxide generator.
With reference to FIG. 1, in a typical Kraft pulp mill 20, wood in the form of chips or sawdust is cooked in a digester 22 with a combination of pulping chemicals known as white liquor to dissolve hemicellulose, lignin, and other extractable materials. White liquor consists primarily of sodium sulfide (Na2S), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and impurities. The digester products, which include cellulosic fibres, dissolved hemicellulose, lignin and extractables, and spent pulping chemicals are separated by filtration in brown stock washer 24. The cellulosic fibres from the wood are retained on the filter, and further processed to pulp. The dissolved hemicellulose, lignin and extractables, and spent pulping chemicals, known as black liquor, are recovered in the Kraft pulp mill chemical recovery cycle.
The first step in the Kraft pulp mill chemical recovery cycle is evaporation in evaporators 26, where black liquor is concentrated by a multi stage evaporation and concentration process. The concentrated black liquor is then burned in a recovery boiler 28. The recovery boiler 28 has two purposes: generating steam for the pulping process, and converting spent chemicals to useful pulping chemicals. The spent chemicals are recovered by dissolving the smelt from the recovery boiler 28 in water in dissolving tank 30 to form green liquor, a solution consisting of mostly dissolved Na2S and Na2CO3.
The green liquor is first treated by clarification or filtration in a green liquor clarifier 32 to remove solids known as dregs. Clarified green liquor is sent to the recausticizing plant 34. In recausticizing, the Na2CO3 in the green liquor is reacted with calcium oxide (CaO) in a causticizing reaction where it is converted to NaOH to form white liquor slurry. The calcium oxide is converted in the reaction to calcium carbonate (CaCO3), which is separated from the white liquor slurry by clarification or filtration in a white liquor clarifier 36, and subsequently burned in a lime kiln 38 to reform calcium oxide.
The clarified or filtered white liquor slurry, known as white liquor, is reused as pulping chemical in the digester 22.
One major source of sulfur losses from the Kraft pulp mill chemical recovery cycle is the electrostatic precipitator (ESP) catch purge. The electrostatic precipitator 40 is used to capture and return solids carried over from the recovery boiler, and a portion of the solids are purged to remove chloride, an impurity, from the chemical recovery cycle. Other chemical losses occur through recovery boiler and lime kiln emissions, liquor lost while removing grits, and knots, liquor spills, sewers in the recausticizing plant, white liquor used in bleach plant scrubber, and SO2 emissions from various sources. (Blackwell and Lincoln, P&P Canada 99:1 1998). These lost chemicals need to be replaced (made up) to maintain the strength of the white liquor used in the digesters.
Several prior art processes have taken advantage of the enrichment of chloride in the electrostatic precipitator catch to facilitate removal of chloride impurities from the chemical recovery cycle by treating the precipitator catch to separate chloride from Na2SO4 and Na2CO3 (see, for example, U.S. Pat. No. 5,922,171). One process described in U.S. Pat. No. 3,833,462 leaches the precipitator catch with sufficient aqueous sulfuric acid solution (which may be spent sulfuric acid from a chlorine dioxide generator) to produce a leached slurry of pH 3-6, thereby converting sodium carbonate to sodium sulfate. The leached solution is filtered to give a cake of anhydrous sodium sulfate and a filtrate enriched in sodium chloride.
One major source of makeup to the chemical recovery cycle is chlorine dioxide generator effluent, which contains sodium sulfate and sulfuric acid. Kraft pulp mills typically include a chlorine dioxide generator 42 to provide chlorine dioxide to a bleach plant 44 if the pulp is to be bleached. The chlorine dioxide generator effluent may be in the form of the acidic salt sodium sesquisulfate. The generator effluent may be used to make up lost sulfur. Some sodium is also recovered with the generator effluent, but the sodium to sulfur (Na:S) ratio is lower than that required in white liquor, so another source of sodium is required. Extra sodium is often made up with caustic soda (NaOH). Acid separation systems to separate sulfuric acid from sodium sulfate are commercially available. The use of these systems results in a higher Na:S ratio in the chemical makeup and reduces the amount of caustic soda which needs to be purchased. The separated sulfuric acid can be used in many places in a Kraft pulp mill, for example in acidification of bleach plant chlorine dioxide stages.
In specific cases where sulfur losses from the Kraft pulp mill chemical recovery cycle are high, separating sulfuric acid for reuse in other areas of the mill results in an excessive loss of sulfur, resulting in the need to purchase additional sulfur chemicals as makeup.